Figures 38 to 41 show projections calculated using the emissions projections based on the 1997 NAEI road transport model, as listed in Table 2. Projections have been calculated for West London, Manchester Town Hall, Cromwell Road and Haringey Roadside. As expected projected concentrations in the early 1990s are lower than those based on the 1996 NAEI and projected concentration in 2005 are higher. The backwards projections based on the 1996 NAEI seem to show at least as good agreement with the measured NO_x concentrations between 1990 and 1998 as the projections based on the revised emissions estimates.

There are indications of a small upward trend in annual mean ozone concentrations over the last 10 years at many rural monitoring sites in the UK (PORG, 1997), presumably as a result of an increase in northern hemisphere background concentrations. While there appears to be a consensus among the ozone modelling community that background concentrations will continue to increase, there is less agreement on the likely magnitude of this increase. Some analyses of the likely impact of such changes in background ozone concentrations on future urban NO₂ concentrations have been carried out by Derwent (1999). A 10 ppb increase in ozone concentrations for all of the hours in a year was found to increase the annual mean NO₂ concentrations by about 7 ppb for a transect across London. The changes in background ozone concentrations are unlikely to be of this magnitude over the next ten years or so and any changes would also be likely to vary from hour to hour and across the country. Significant increases in background ozone concentrations would clearly lead to annual mean NO₂ concentrations higher than those projected using the NO_x to NO₂ relationship based on 1997 measurement data presented here.

Chapter 5         Chapter 7